1. Field of the Invention
The present invention relates to fiber optic gyroscopes. More particularly, this invention pertains to a potted sensor coil design for use in high vibration environments, and in rapidly-changing temperature environments.
2. Description of the Prior Art
An interferometric fiber optic gyroscope comprises the following main components: (1) a light source, (2) two beamsplitters (fiber optic directional coupler and/or integrated-optics Y-junctions) to satisfy the requirement of a "minimum reciprocal configuration" (S. Ezekiel and M. J. Arditty, Fiber Optic Rotation Sensors New York, Springer-Verlag p. 2-26 1982), (3) a fiber optic sensing coil made of either polarization maintaining (PM) fiber or made of low-birefringence (standard telecommunications) fiber, (4) a polarizer (and sometimes one or more depolarizers), and (5) a detector. Light from the light source is split by the loop beamsplitter into copropagating and counterpropagating waves travelling in the sensing coil. The associated electronics measures the phase relationship between the two interfering, counter-propagating beams of light that emerge from opposite ends of the coil. The difference between the phase shifts experienced by the two beams is proportional to the rate of rotation of the platform to which the instrument is fixed, due to the well-known Sagnac effect.
Environmental factors can affect the measured phase shift difference between the counterpropagating beams, thereby introducing a bias error. Such environmental factors include variables such as temperature, vibration (acoustical and mechanical) and magnetic fields. In general, such factors are both time-varying and unevenly distributed throughout the coil. These environmental factors induce variations in the optical light path that each counterpropagating wave encounters as it travels through the coil. The phase shifts induced upon the two waves are unequal, producing a net undesirable phase shift which is indistinguishable from the rotation-induced signal.
One approach to attain a reduction of sensitivities arising from environmental factors has involved the use of various symmetric coil winding configurations. In such coils, the windings are arranged so that the geometrical center of the coil is located at the innermost layer while the two ends of the coil are located at the outermost layers.
N. Frigo has proposed the use of particular winding patterns to compensate for non-reciprocities "Compensation of Linear Sources of Non-Reciprocity in Sagnac Interferometers". Fiber Optics and Laser Sensors I, Proc. SPIE Vol. 412 p. 268 (1983). Furthermore, U.S. Pat. No. 4,793,708 of Bednarz entitled "Fiber Optic Sensing Coil" teaches a symmetric fiber optic sensing coil formed by dualpole or quadrupole winding. The coils described in that patent exhibit enhanced performance over the conventional helix-type winding.
U.S. Pat. No. 4,856,900 of Ivancevic entitled "Quadrupole-Wound Fiber Optic Sensing Coil and Method of Manufacture Thereof" teaches an improved quadrupole-wound coil in which fiber pinching and microbends due to the presence of pop-up fiber segments adjacent the end flanges are overcome by replacing such pop-up segments with concentrically-wound walls of turns for climbing between connecting layers. Both of the aforementioned United States patents are the property of the assignee herein.
While appropriate coil winding techniques minimize some of the bias errors found in the output of a fiber optic gyro, they are not capable of eliminating all of such biases. In particular, the design of the gyro sensor coil can impact the gyro's random walk, bias stability, bias temperature sensitivity, bias temperature-ramp sensitivity, bias vibration sensitivity, bias magnetic sensitivity, scale factor temperature sensitivity, scale factor linearity and input axis temperature sensitivity.
It is recognized that potting the windings of a sensor coil within a matrix of adhesive material is advantageous as it facilitates the precision of coil winding. Furthermore, it was disclosed in pending U.S. Pat. application 07/938,294 of co-inventors Amado Cordova, Donald J. Bilinski, Samuel N. Fersht, Glenn M. Surabian, John D. Wilde and Paul A. Hinman entitled "Sensor Coil For Low Bias Fiber Optic Gyroscope", now U.S. Pat. No. 5,321,593 that the composition of the potting material can have a significant impact upon the bias vibration sensitivity of the fiber optic gyro due to a non-reciprocal phase shift between the light waves counterpropagating within the coil as a result of changes in fiber length and refractive index brought about by vibration dynamic strains.
The referenced United States patent application discloses a sensor coil whose design incorporates a number of features for minimizing the aforesaid environmental factors. Among the issues identified and addressed in that patent application is the relationship between the modulus of elasticity of the potting material composition of an encapsulated sensor coil and vibration-induced bias..
Generally, it is disclosed in that application that gyro performance in terms of vibration-induced bias is significantly improved by the use of potting material possessing a high modulus, of elasticity or Young's modulus although not so high as to produce other problems related to gyro operation at temperatures significantly removed from the curing temperature of the potting material such as temperature related coil cracking, h-parameter (polarization cross-coupling) degradation if the coil is of PM-fiber composition, and large bias temperature-ramp sensitivity. Polymers are attractive candidates for the adhesive potting material due to such common properties as substantial impermeability to moisture and the like. Sensor coils have been fabricated of polymers that incorporate the teachings of the referenced patent application. For example, coils encapsulated in the UV-curable acrylate-adhesive marketed under the trademark "NORLAND 65" have demonstrated adequate vibration bias characteristics. However, when cycled through a temperature range that includes the gyro's operational range, such coils have exhibited some disturbing, temperature-related anomalies. These include so-called "bias spiking" and "bias crossing". Each of such phenomena can significantly hamper the successful operation of the gyro. A bias spike may be of sufficient magnitude to take a gyro out of specifications while bias crossing can effectively render the ability to model out the bias error impossible or impractical.